1. Field of the Invention
The present invention relates to a process for preparing raw particles of cadmium sulfide for electrophography.
2. Description of the Prior Art
A conventional process hitherto used for preparing raw particles of cadmium sulfide is to introduce hydrogen sulfide gas into a solution containing cadmium ion such as a solution of cadmium sulfate so as to deposit cadmium sulfide particles from the solution. The cadmium sulfide particles prepared by such a conventional process are raw particles for use in the preparation of final cadmium sulfide particles as a photoconductive material for electrophotography. To prepare the final cadmium sulfide particles for electrophotography, it is usually necessary to further treat the above-mentioned raw particles. Various treating methods have been known and used for this purpose. According to one typical treating method, an impurity having a sensitizing effect of the type so-called donor and acceptor is added to the raw particles and the mixture is heated to a high temperature ranging from 400.degree. C. to 700.degree. C. This treatment has the effect to diffuse the impurity into the raw particles and activate them so that sensitized cadmium sulfide particles may be formed. The sensitized cadmium sulfide particles thus obtained are used as a photoconductive material for electrophotography.
Another conventional method used for preparing raw particles of cadmium sulfide is to use a solution containing cadmium ion and ion of impurity element. This solution is reacted with hydrogen sulfide to form raw particles of cadmium sulfide that preliminarily contains impurity. Since the raw particles thus prepared have already contained the necessary impurity, it is allowed to obtain the aimed cadmium sulfide particles in which impurity is diffused i.e. the aimed sensitized particles by an after-treatment with heat, without incorporating any impurity at the treatment. Also, the sensitized cadmium sulfide particles can be used as a photoconductive material for electrophotography. The latter mentioned preparing method has the advantage that the heat treatment can be carried out at a relatively low temperature compared with the former since the necessary impurity has already been contained in the raw particles before the heat treatment. Furthermore, according to the latter method, a better efficiency is attainable in dispersing the impurity.
However, it has been found that these conventional processes have, in common, some drawbacks as to the preparation of the raw particles.
One of the important disadvantages is found in the fact that the final photoconductive cadmium sulfide particles for electrophotography do not exhibit the same and constant electric characteristic. As known to those skilled in the art, it is important that all of photoconductive cadmium sulfide particles for electrophotography obtained by treating raw particles prepared under the same preparing condition should exhibit the same and constant electrophotographic characteristic. This is an essential requirement to make the produce marketable. But, it is impossible to produce good product particles enough to satisfy the requirement according to the conventional processes as described above. In particular, the first mentioned conventional process can not satisfy it.
As another drawback of the conventional processes, it is pointed out that the final photoconductive cadmium sulfide particles for electrophotography obtained by the heat treatment of the raw particles prepared by the conventional processes are poor in moisture resisting property. They are apt to absorb atmospheric moisture and therefore the electrophotographic characteristic of the cadmium sulfide particles changes very easily in accordance with the change in atmospheric humidity. With such cadmium sulfide particles it is difficult to make a good electrophotographic photosensitive material stable against moisture.